This article is being written as apple bud movement in Hawke’s Bay is getting underway. Pink is showing in blocks that have been treated with dormancy breakers. It is before the start of the chemical thinning season now but by the time you read this, it will be done if not dusted. Strategies will be implemented and treatments applied but the final outcome won’t yet be fully revealed. So that’s the context and timeline for this brief chat about chemical thinning – where to begin? A good place to start is considering the objectives with chemical thinning of apples. Table 1 is an attempt to list them in some priority order:
Table 1. Thinning objectives
1. Good return bloom – next year
2. A full crop – this year 3. Fruit retained on best sites 4. Uniform crop load over the tree 5. A small and economical hand thin job Achieving all five objectives (or even scarier if they are called key performance indicators) is a tall order when applied to every block on every orchard. If we agree that points one and two are top priority – uniform return bloom next year and sufficient fruit numbers for a full crop this year, its sounding more doable. If points three and four are considered as components of a full crop, then maybe its sounding easier still. Maybe it’s the final point, the financial motivation for minimal hand thin labour costs that is the cause for going ‘too hard’ and overdoing it at the expense of objectives one to four? If fruit borne on the best spur sites have been ‘blown off’ by ‘hot brews’ and the easily thinned bottoms turn out shy on crop, we’ve failed at points three, four and probably two!
SEASON 2016-7 WAS AN OUTLIER
The previous season, 2016-7, was a climatic outlier over the critical stage of fruitlet thinning. Both Hawkes Bay and Nelson experienced unusually low solar radiation and high night temperatures for about seven days when the fruit size of many blocks was ‘in the zone’ (8-16mm fruitlet diameter). These conditions coincided with a big increase in the use of a new chemistry for apple thinning. We’re talking about the photosynthetic inhibitor, metamitron, which has a totally different mode of action from the products we have decades of experience with.
The standard bloom and fruitlet thinner products are driven by temperature – the hotter it is over the days following application, the greater the efficacy. It’s a response we understand and look for in planning our timing of application.
So, given a new product with a different driver, namely carbohydrate deficit, a few ‘overthin’ surprises were on the cards.
CARBON BALANCE AND TREE SENSITIVITY
The Carbohydrate Model developed at Cornell University, New York, provides growers there with an on-line decision support tool to guide chemical thinning strategy and implementation. The model uses current and forecast maximum and minimum temperatures and solar radiation to predict the tree’s sensitivity to natural drop and responses to chemical thinning.
More fruitlet drop and easier thinning occurs following cloudy conditions and high (especially night) temperatures. Trees are more sensitive to thinning when their carbohydrate balance is lowered by reduced photosynthetic production and higher respiration demand (or cost) resulting from these conditions. This was the situation in early November, 2016 – see figure 2 for the situation in normally sunny Nelson.
We also learned how the effects of ‘artificially induced shade’ i.e. the application of a photosynthetic inhibitor, could combine with natural shade the climatic pre-conditioning to give excessive thinning in the less well-lit zones of trees. This has prompted a ‘re-think’ around the set-up and calibration of spraying equipment.
UNIFORM CROP OVER THE WHOLE TREE
Fruition put this rather technical subject to the contestants at T&G’s annual Hortisports day in Hastings in August. Their challenge, with no warning and in front of a large crowd, was to ‘spray’ their tree with blue paint with a coverage pattern that would result in a uniform crop over the whole tree. I think that the young contestants did an awesome job in a high pressure situation. They showed a great understanding of the factors at play.Three samples of their ‘calibration artwork’ are shown in Figure 3. Successful implementation in the field presents the challenge of turning a target into a result.
FUJI – A TOUGHER NUT TO CRACK
It would be wrong to create the impression that over-thinning was the only issue. Apple varietal differences are pronounced and Fuji stands out as a challenge being both difficult to thin and susceptible to biennial bloom behavior.Add in its tendency to bloom over an extended time which results in a wide range of fruitlet sizes and you have a challenging customer. While dormancy breakers can help by compacting bloom, the new
“Will 2017 see large fruit sets and expensive hand thinning jobs resulting from overly timid thinning programmes? We should have a better idea by the time you read this.”
chemistry struggled to produce stunning results even at higher rates and given the climatic context. There is still plenty more to learn here.
TO BLOOM THIN OR NOT – THAT IS THE QUESTION
The introduction of powerful fruitlet thinning chemistry (metamitron) that has been shown to thin as much alone as when pre-ceeded with bloom thinners raises a question. Do we need to bother with bloom thinners anymore? Its fair to say that sometimes bloom thinners don’t work, sometimes they do harm and sometimes they work well. A key factor is how important bloom thinners are in achieving our first objective – good return bloom.
STRINGS TO THE BOW
Its great to have two groups of products that respond to different drivers – warm temperatures for the plant growth regulators and carbohydrate deficits for the photosynthetic inhibitors. It does give the opportunity to select the group best suited to the climatic conditions. Not that I’m suggesting that these decisions are a ‘walk in the park’.
NO TWO SEASONS THE SAME
The odds are that we will have experienced more normal conditions over bloom and fruitset in 2017 than we did in 2016. That would mean more sunshine and cooler nights resulting in more positive carbohydrate balances and lower sensitivity to fruit drop and chemical thinning. It would be understandable if many growers were inclined to be cautious following unwelcome surprises in 2016. Will 2017 see large fruit sets and expensive hand thinning jobs resulting from overly timid thinning programmes? We should have a better idea by the time you read this.